When a water based pigment ink is colored navy blue, it has so far been carried out to use a navy blue colorant or blend a plurality of colorants to prepare a navy blue color. A navy blue colorant includes Prussian blue, cobalt blue (cobalt aluminum oxide: C. I. 77346) and the like, and a blended colorant includes a combination of carbon black and copper phthalocyanine pigment. The latter method of blending colors can be an effective means when a viscosity of an ink in a ballpoint pen is 50 to 1000 mPa·s, but if a viscosity of the ink is 1 to 10 mPa·s, there is a strong possibility of occurring selective precipitation of the colorants and the stability with the passage of time is poor.
Inorganic pigments such as Prussian blue and cobalt blue have a weak coloring power as compared with those of organic pigments, and an inorganic pigment having a poor alkali resistance such as Prussian blue is further reduced in a coloring power particularly when a pH of the ink is set 7 to 10. This requires to increase the blending amount of the pigment in the ink, and as a result, when the ink is filled into an ink tank of a ballpoint pen in which the ink in the ink tank can visually be observed from the outside, a color of the ink in the ink tank which is visually observed from the outside is very different from that of the drawn lines, so that a color of the drawn lines can not be recognized from the apparent ink color.
Further, increasing a blending amount of a pigment in an ink brings about marked tendency to cohere the pigment in storing over a long period of time, and particularly when inorganic pigments are used, the problem that a ballpoint pen tip is worn to a large extent by writing is caused, which leads to a reduction in writing feeling and a writing distance.
Disclosed in Japanese Patent Application Laid-Open No. Hei 10-95947(1998) is an ink containing lithium cobalt titanate in order to make a color of the drawn lines cobalt blue (light blue). However, such inorganic pigment causes an ink to be poor in stability with the passage of time because of the factors as described above and the writing feeling and the writing distance of the ink are lowered, particularly when a viscosity of the ink is 1 to 10 mPa·s.
An object of the present invention is to provide a water based ink composition for a ballpoint pen which makes it possible to visually observe a color of the ink in an ink tank and a color of the drawn lines to be almost the same navy blue color when the ink is filled into a ballpoint pen tank in which the ink in the ink tank can visually be observed from the outside, and which is not changed even in storing over a long period of time and is excellent in lubricity.